Recently, there has been growing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, laptop computers and even electric cars, efforts have increasingly been made towards the research and development of electrochemical devices.
In this aspect, electrochemical devices have attracted the most attention. Among them, the development of rechargeable secondary batteries has been the focus of particular interest. In recent years, extensive research and development has been conducted to design new electrodes and batteries for the purpose of improving capacity density and specific energy of the batteries.
Assessing and ensuring the safety of electrochemical devices is very important. One of the most important considerations is that electrochemical devices should not cause damage to users in the event of malfunction, and for this purpose, Safety Standards impose strict regulations on ignition and explosion of electrochemical devices. In the safety characteristics of electrochemical devices, electrochemical devices have a high risk of explosion in the event of overheat or thermal runaway of an electrochemical device or penetration of a separator. Particularly, a polyolefin porous substrate commonly used for a separator of an electrochemical device shows serious thermal contraction behaviors at temperature less than or equal to 150° C. due to material characteristics and procedural characteristics in the manufacturing process including stretching, causing a short circuit between a cathode and an anode.
To solve the problem, a composite separator with a porous coating layer has been proposed in which a slurry including inorganic particles or organic particles and a binder polymer is coated on at least one surface of a polyolefin porous substrate having a plurality of pores. In the composite separator, the inorganic/organic particles in the coating layer of the polyolefin porous substrate serve as a support to maintain a mechanical shape of the coating layer, and thus prevent the polyolefin porous substrate from thermally contracting when a lithium ion battery is overheated.
Referring to FIG. 1, a process for manufacturing such a separator according to a related art includes the steps of extruding a polyolefin resin composition, stretching the extruded resin composition to obtain a film on a sheet, extracting a diluent from the obtained separator to obtain a porous film, heat setting the porous film, winding/slitting the heat-set porous film, unwinding, applying a coating slurry, drying the coating slurry, secondary winding/slitting, and packing a product.
According to the process according to the related art, there is a limitation that the heat setting process should be performed at temperature not causing the polyolefin film to be melted. Also, due to a risk of destruction of structural stability after coating and drying of the slurry on the porous substrate, it is difficult to perform an additional heat setting process.
Also, Japanese Patent No. 5543715 discloses a method of manufacturing a separator for a non-aqueous electrolyte battery including (i) melt-kneading polyolefin resin and a diluent, or polyolefin resin, a diluent, and an inorganic agent and extruding the mixture, (ii) stretching the obtained extruded product, (iii) extracting the diluent or the diluent and the inorganic agent. However, this does not correspond to a method involving coating a slurry including inorganic particles and others after forming a porous substrate, and does not provide a description of an order of slurry coating and heat setting steps and their specific conditions.
Also, Korean Patent Registration No. 10-0406690 discloses that a multicomponent film used as a separator for an electrochemical device is manufactured by a method including i) providing a polymer support film; ii) dissolving gelling polymer in a solvent to prepare a gelling polymer solution; iii) forming a gelling polymer layer from the gelling polymer solution of the step ii) on one surface or both surfaces of the support film of the step i) to manufacture a multilayer film; and iv) stretching and heat setting the multilayer film of the step iii). However, this paper just teaches coating the gelling polymer solution on the porous substrate to form the gelling polymer layer, and does not disclose a step of coating a slurry including organic particles or inorganic particles to form a porous coating layer. Also, because this paper involves coating the gelling polymer layer on the polymer support film and then stretching and heat setting of the obtained multilayer film, in the case of a composite film with a porous coating layer including organic particles and/or inorganic particles, cracking may occur in the coating layer in a TD direction during stretching after coating, which is a limitation on applications.